Nucleic acid-based assays can enable highly specific and sensitive detection of nucleic acid analytes from a variety of sources, including clinical, industrial, environmental, and food sources. These assays can be used to determine or monitor for the presence or amount of biological antigens (e.g., prions), cell abnormalities, disease states, and disease-associated pathogens, including parasites, fungi, bacteria and viruses present in a host organism or sample. Nucleic acid-based assays may be qualitative or quantitative, with the quantitative assays providing useful information to practitioners for evaluating the extent of infection or disease or to determine the state of a disease over time. Quantitative assays can also be used, for example, to assess the effectiveness of a therapeutic treatment program or, alternatively, to determine the extent of an infection or contamination by a particular organism or virus.
All nucleic acid-based assay formats involve a number of process steps leading to the identification, detection or quantification of one or multiple target nucleic acids in a sample. When necessary, the specifically targeted nucleic acid sequences of a nucleic acid-based assay may be unique to an identifiable group of organisms (as used herein, the term “organisms” is inclusive of viruses), where the group is defined by at least one shared nucleic acid sequence that is common to all members of the group and that is specific to that group. (A “group” of organisms is generally a phylogenetic grouping of organisms, such as a strain, species or genus of organisms and may be limited to a single organism.) Generally, the uniqueness of the targeted nucleic acid sequence or sequences need only be limited to the particular sample type being assayed (e.g., a human sample versus an industrial or environmental sample). Nucleic acid-based methods and means for detecting individual and groups of organisms are disclosed by Kohne, “Method for Detection, Identification and Quantitation of Non-Viral Organisms,” U.S. Pat. No. 4,851,330, and Hogan et al., “Nucleic Acid Probes for Detection and/or Quantitation of Non-Viral Organisms,” U.S. Pat. No. 5,541,308.
After determining what organisms are to be targeted by an assay, the first step is to select or design a probe which exhibits specificity for a nucleic acid sequence belonging to those organisms which define the group. Nucleic acid-based assays can be designed to detect either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), including ribosomal RNA (rRNA), transfer RNA (tRNA) or messenger RNA (mRNA). For prokaryotic and eukaryotic organisms, rRNA or the encoding DNA (rDNA) is generally a preferred target for detection. Ribosomal RNA sequences are particularly preferred targets for non-amplified, nucleic acid-based assays because of their relative abundance in cells, and because rRNA contains regions of sequence variability that can be exploited to design probes capable of distinguishing between even closely related organisms. Viruses, which do not contain ribosomal nucleic acid, and cellular changes are often best detected by targeting DNA, RNA, or a messenger RNA (mRNA) sequence. See, e.g., McDonough et al., “Detection of Human Immunodeficiency Virus Type 1, U.S. Pat. No. 6,649,749; and Fradet et al., “Methods to Detect Prostate Cancer in a Sample,” U.S. Patent Application Publication No. US 2005-0282170 A1. Such viruses may include positive-strand RNA viruses (e.g., hepatitis C virus), where the RNA genome is mRNA, negative-strand RNA viruses (e.g., influenza viruses), retroviruses (e.g., human immunodeficiency virus), single-stranded DNA viruses (e.g., parvoviruses), and double-stranded DNA viruses (e.g., adenoviruses), which would require a melting step to render the double-stranded target region sufficiently single-stranded for amplification or detection. When the focus of a nucleic acid-based assay is the detection of a genetic abnormality, then the probes are usually designed to detect identifiable changes in the genetic code, an example of which is the abnormal Philadelphia chromosome associated with chronic myelocytic leukemia. See, e.g., Stephenson et al., “Deoxynucleic Acid Molecules Useful as Probes for Detecting Oncogenes Incorporated Into Chromosomal DNA,” U.S. Pat. No. 4,681,840.
When performing a nucleic acid-based assay, preparation of the sample is necessary to release and stabilize target nucleic acids which may be present in the sample. Sample preparation can also serve to eliminate nuclease activity and remove or inactivate potential inhibitors of nucleic acid amplification (discussed below) or detection of the target nucleic acids. See, e.g., Ryder et al., “Amplification of Nucleic Acids From Mononuclear Cells Using Iron Complexing and Other Agents,” U.S. Pat. No. 5,639,599, which discloses methods for preparing nucleic acid for amplification, including the use of complexing agents able to complex with ferric ions released by lysed red blood cells. The method of sample preparation can vary and will depend in part on the nature of the sample being processed (e.g., blood, urine, stool, pus or sputum). When target nucleic acids are being extracted from a white blood cell population present in a diluted or undiluted whole blood sample, a differential lysis procedure is generally followed. See, e.g., Ryder et al., “Preparation of Nucleic Acid From Blood,” European Patent Application No. 0 547 267 A2. Differential lysis procedures are well known in the art and are designed to specifically isolate nucleic acids from white blood cells, while limiting or eliminating the presence or activity of red blood cell products, such as heme, which can interfere with nucleic acid amplification or detection. Other lytic methods are disclosed by, for example, Cummins et al., “Methods of Extracting Nucleic Acids and PCR Amplification Without Using a Proteolytic Enzyme,” U.S. Pat. No. 5,231,015, and Clark et al., “Methods for Extracting Nucleic Acids From a Wide Range of Organisms by Nonlytic Permeabilization,” U.S. Pat. No. 5,837,452; and Cunningham et al., “Compositions, Methods and Kits for Determining the Presence of Cryptosporidium Organisms in a Test Sample,” U.S. Patent Application Publication No. US 2002-0055116 A1.
To purify the sample and remove nucleases and other materials capable of interfering with amplification or detection, the targeted nucleic acid can be isolated by target-capture means using a “capture probe” which binds the target nucleic acid and is or becomes either directly or indirectly bound to a solid substrate, such as a magnetic or silica particle. See, e.g., Ranki et al., “Detection of Microbial Nucleic Acids by a One-Step Sandwich Hybridization Test,” U.S. Pat. No. 4,486,539; Stabinsky, “Methods and Kits for Performing Nucleic Acid Hybridization Assays,” U.S. Pat. No. 4,751,177; Boom et al., “Process for Isolating Nucleic Acid,” U.S. Pat. No. 5,234,809; Englehardt et al., “Capture Sandwich Hybridization Method and Composition,” U.S. Pat. No. 5,288,609; Collins, “Target and Background Capture Methods and Apparatus for Affinity Assays,” U.S. Pat. No. 5,780,224; and Weisburg et al., “Two-Step Hybridization and Capture of a Polynucleotide,” U.S. Pat. No. 6,534,273. When the solid support is a magnetic particle, magnets in close proximity to the reaction receptacle are used to draw and hold the magnetic particles to the side of the receptacle, thereby isolating any bound nucleic acid within the reaction receptacle. Other methods for isolating bound nucleic acid in a reaction receptacle include centrifugation and immobilizing the capture probe on the reaction receptacle. See, e.g., Boom et al., supra, and Urdea, “Polynucleotide Capture Assay Employing in Vitro Amplification,” U.S. Pat. No. 5,200,314. Once the bound nucleic acid is thus isolated, the bound nucleic acid can be separated from unbound nucleic acid and other cellular and sample material by aspiring the fluid contents of the reaction receptacle and optionally performing one or more wash steps with a wash solution.
In most cases, it is desirable to amplify the target sequence. Nucleic acid amplification involves the use of nucleic acid polymerases to enzymatically synthesize nucleic acid amplification products (copies) containing a sequence that is either complementary or homologous to the template nucleic acid sequence being amplified. The amplification products may be either extension products or transcripts generated in a transcription-based amplification procedure. Examples of nucleic acid amplification procedures practiced in the art include the polymerase chain reaction (PCR), strand displacement amplification (SDA), loop-mediated isothermal amplification (LAMP) ligase chain reaction (LCR), immuno-amplification, and a variety of transcription-based amplification procedures, including transcription-mediated amplification (TMA), nucleic acid sequence based amplification (NASBA), and self-sustained sequence replication (3SR). See, e.g., Mullis, “Process for Amplifying, Detecting, and/or Cloning Nucleic Acid Sequences,” U.S. Pat. No. 4,683,195; Walker, “Strand Displacement Amplification,” U.S. Pat. No. 5,455,166; Notomi et al., “Process for Synthesizing Nucleic Acid,” U.S. Pat. No. 6,410,278; Birkenmeyer, “Amplification of Target Nucleic Acids Using Gap Filling Ligase Chain Reaction,” U.S. Pat. No. 5,427,930; Cashman, “Blocked-Polymerase Polynucleotide Immunoassay Method and Kit,” U.S. Pat. No. 5,849,478; Kacian et al., “Nucleic Acid Sequence Amplification Methods,” U.S. Pat. No. 5,399,491; Malek et al., “Enhanced Nucleic Acid Amplification Process,” U.S. Pat. No. 5,130,238; and Lizardi et al., BioTechnology, 6:1197 (1988). Nucleic acid amplification is especially beneficial when the amount of target sequence present in a sample is very low. By amplifying the target sequences and detecting the synthesized amplification product, the sensitivity of an assay can be vastly improved, since fewer target sequences are needed at the beginning of the assay to ensure detection of the targeted nucleic acid sequences.
Detection of a target nucleic acid requires the use of a probe having a nucleotide base sequence which binds to a target sequence contained within the target nucleic acid or, alternatively, amplification product containing the target sequence or its complement. Probes useful for distinguishing between sources of nucleic acid are selected or designed such that they do not detectably bind to nucleic acid from non-target organisms which may be present in the sample under the selected assay conditions. While probes may include non-nucleotide components, the target binding portion of a probe will include DNA, RNA and/or analogs thereof in order to effect hybridization to the target sequence or its complement. See, e.g., Becker et al., “Modified Oligonucleotides for Determining the Presence of a Nucleic Acid Analyte in a Sample,” U.S. Patent Application No. US 2003-0036058 A1 (discloses the use of 2′-O-methyl modified probes); and Nielsen et al., “Peptide Nucleic Acids,” U.S. Pat. No. 5,539,082 (discloses the use of probes having a 2-aminoethylglycine backbone which couples the nucleobase subunits by means of a carboxylmethyl linker to the central secondary amine). For detection purposes, probes may include a detectable label, such as a radiolabel, fluorescent dye, biotin, enzyme or chemiluminescent compound, where the label may be provided either before, during or after hybridization to the probe to the target sequence or its complement. See, e.g., Higuchi, “Homogenous Methods for Nucleic Amplifications and Detection,” U.S. Pat. No. 5,994,056 (discloses the use of intercalating agents such as eithidium bromide); and Urdea et al., “Solution Phase Nucleic Acid Sandwich Assays Having Reduced Background Noise,” U.S. Pat. No. 5,635,352 (discloses use of label probes for binding to cruciform structures containing a target nucleic acid).
Nucleic acid-based assays may be based on a homogenous or a heterogenous format. One form of a heterogenous assay involves preferentially binding a probe:target complex to a solid support, such as glass, minerals or polymeric materials, and removing any unbound probe prior to detection. In an alternative approach, it is the unbound probe which is associated with the solid support while probe complexed with the target sequence remains free in solution and can be separated for detection. Homogenous assays generally take place in solution without a solid phase separation step and commonly exploit chemical differences between a probe free in solution and a probe which has formed part of a target:probe complex. An example of a homogenous assay is the Hybridization Protection Assay (HPA), the particulars of which are disclosed by Arnold et al., “Homogenous Protection Assay,” U.S. Pat. No. 5,639,604. Detection in HPA is based on differential hydrolysis which permits specific detection of an acridinium ester-labeled probe hybridized to the target sequence or its complement. See, e.g., Arnold et al., “Protected Chemiluminescent Labels,” U.S. Pat. No. 4,950,613; Campbell et al., “Chemilunescent Acridium Labelling Compounds,” U.S. Pat. No. 4,946,958; Arnold et al., “Acridinium Ester Labelling and Purification of Nucleotide Probes,” U.S. Pat. No. 5,185,439; and Arnold et al, “Linking Reagents for Nucleotide Probes,” U.S. Pat. No. 5,585,481. This detection format includes both a hybridization step and a selection step. In the hybridization step, an excess of acridinium ester-labeled probe is added to the reaction receptacle and permitted to anneal to the target sequence or its complement. Following the hybridization step, label associated with unhybridized probe is rendered non-chemiluminescent in the selection step by the addition of an alkaline reagent. The alkaline reagent specifically hydrolyzes only that acridinium ester label associated with unhybridized probe, leaving the acridinium ester of the probe:target hybrid intact and detectable. Chemiluminescence from the acridinium ester of the hybridized probe can then be measured using a luminometer and signal is expressed in relative light units or RLU.
Other homogenous assays include those disclosed by the following: Gelfand et al., “Reaction Mixtures for Detection of Target Nucleic Acids,” U.S. Pat. No. 5,804,375; Nadeau et al., “Detection of Nucleic Acids by Fluorescence Quenching,” U.S. Pat. No. 5,958,700; Tyagi et al., “Detectably Labeled Dual Conformation Oligonucleotide Probes, Assays and Kits,” U.S. Pat. No. 5,925,517; Morrison, “Competitive Homogenous Assay,” U.S. Pat. No. 5,928,862; and Becker et al., “Molecular Torches,” U.S. Pat. No. 6,849,412. These patents each describe unimolecular or bimolecular probes which may be used to determine the amount of a target nucleic acid in an amplification procedure in real-time, where signal changes associated with the formation of probe:target complexes are detected during amplification and used to calculate an estimated amount of a target nucleic acid present in a sample. Algorithms for calculating the quantity of target nucleic acid originally present in a sample based on signal information collected during an amplification procedure include that disclosed by Wittwer et al., “PCR Method for Nucleic Acid Quantification Utilizing Second or Third Order Rate Constants,” U.S. Pat. No. 6,232,079; Sagner et al., “Method for the Efficiency-Corrected Real-Time Quantification of Nucleic Acids,” U.S. Pat. No. 6,691,041; McMillan et al., “Methods for Quantitative Analysis of a Nucleic Acid Amplification Reaction,” U.S. Pat. No. 6,911,327; and Chismar et al., “Method and Algorithm for Quantifying Polynucleotides,” U.S. Provisional Application No. 60/693,455, which enjoys common ownership herewith.
After the nucleic acid-based assay is run, and to avoid possible contamination of subsequent amplification reactions, the reaction mixture can be treated with a deactivating reagent which destroys nucleic acids and related amplification products in the reaction receptacle. Such reagents can include oxidants, reductants and reactive chemicals which modify the primary chemical structure of a nucleic acid. These reagents operate by rendering nucleic acids inert towards an amplification reaction, whether the nucleic acid is RNA or DNA. Examples of such chemical agents include solutions of sodium hypochlorite (bleach), solutions of potassium permanganate, formic acid, hydrazine, dimethyl sulfate and similar compounds. More details of deactivation protocols can be found in Dattagupta et al., “Method and Kit for Destroying the Ability of Nucleic Acid to Be Amplified,” U.S. Pat. No. 5,612,200, and Nelson et al., “Reagents, Methods and Kits for Use in Deactivating Nucleic Acids,” U.S. Patent Application Publication No. US 2005-0202491 A1.
Given the large number of complex steps associated with nucleic acid-based amplification assays, and the different processing and equipment requirements of each type of amplification assay, a need exists for an automated system capable of processing the contents of a plurality of reaction receptacles according to different amplification assay protocols, and most especially for performing both real-time and end-point amplification assays on the same platform and/or within a self-contained housing. Real-time amplification assays involve periodically determining the amount of targeted amplification products as the amplification reaction is taking place, thereby making it easier to provide quantitative information about target nucleic acids present in a sample, whereas end-point amplifications determine the amount of targeted amplification products after the amplification reaction has occurred, generally making them more useful for providing qualitative information about target nucleic acids. To improve flow through and, thereby, to reduce the amount of time needed to process large volumes of samples, there is also a need for a system capable of continuously processing the contents of multiple reaction receptacles according to a real-time amplification protocol without having to interrupt the system to manually or automatically load a new batch of reaction receptacles for processing. Additionally, there is a need for a reagent and method for reducing the amount of amplification inhibitors in reaction receptacles that could affect qualitative or quantitative determinations.